Key Highlights
- Discovery of galaxies as ‘island universes’ beyond the Milky Way
- Spacetime curvature due to mass and energy
- Gravitational waves and light bending as consequences
- Redshift observation: Light wavelength alteration with distance
It’s been almost 100 years since humanity first reached a revolutionary conclusion about the nature of our Universe: space itself cannot and does not remain static, but rather evolves with the passage of time. One of the most unsettling predictions of Einstein’s general relativity is that any Universe — so long as it’s uniformly (or almost uniformly) filled with one or more species of matter, radiation, or energy — cannot remain the same over time. Instead, it must either expand or contract, something initially derived independently by three separate people: Alexander Friedmann (1922), Georges Lemaitre (1927), and Howard Robertson (1929), and was later generalized by Arthur Walker (1936).
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Galaxies: Windows to Cosmic Evolution
Right at around the same time, starting in 1923, observations began to show that the spirals and ellipticals in our sky were actually galaxies: ‘island universes’ that were well outside of our own Milky Way. With new, more powerful measurements, we could determine that the farther away a galaxy was from us, the greater the arriving light in our instruments was redshifted, or observed at longer wavelengths, compared to the light that was initially emitted. It was as though the very act of journeying through space altered the wavelength of that traveling light.
General Relativity: Curving the Fabric of Spacetime
An animated look at how spacetime responds as a mass moves through it helps showcase exactly how, qualitatively, it isn’t merely a sheet of fabric. Instead, all of 3D space itself gets curved by the presence and properties of matter and energy within the Universe. Multiple masses in orbit around one another will cause the emission of gravitational waves, while any light passing through a region that contains this distorted spacetime will be bent, distorted, and possibly magnified by the effects of curved space.
Exploring the Foundation: General Relativity
The starting point of our conversation has to be general relativity: our modern theory of gravity first put forth by Einstein. General relativity, at its core, is a framework that relates two things that might not obviously be related:
- the amount, distribution, and types of energy — including matter, antimatter, dark matter, radiation, neutrinos, and anything else you can imagine — that are present all throughout the Universe,
- and the geometry of the underlying spacetime, including whether and how it’s curved and whether and how spacetime itself will evolve.
Echoes of Einstein: Gravitational Waves and Time Dilation
For example, if we go back to the black hole example (although it applies to any mass), we can calculate how severely space is curved in that black hole’s vicinity. If the black hole is spinning, we can calculate how significantly space is ‘dragged’ along with the black hole due to the presence of the black hole’s angular momentum. If we then measure what happens to objects in the vicinity of those objects, we can compare what we see with the predictions of general relativity. In other words, we can actually perform experiments to determine whether space curves in the way Einstein’s theory tells us it ought to in a wide variety of ways. As soon as the first conflict arises — as soon as a single observation is inconsistent with Einstein’s predictions — we will have a compelling case for extending the theory of gravity beyond Einstein’s general relativity.
What about the Universe’s expansion?
The simplest evolving quantity we can consider in an expanding Universe is density. If our Universe is filled with ‘stuff’, then as the Universe expands, the volume of the space that this ‘stuff’ occupies will increase.
We normally think about matter as the ‘stuff’ we’re thinking about. Matter is, at its simplest level, a fixed amount of massive ‘stuff’ that lives within space. As the Universe expands, the total amount of stuff remains the same, but the total amount of space for the stuff to live within increases. For matter, density is just mass divided by volume, and so if your mass stays the same (or, for things like atoms, if the number of particles stays the same) while your volume grows, your density should go down. When we do the general relativity calculation, that’s exactly consistent with what we find for matter.
But even though we have multiple types of matter in the Universe — normal matter, black holes, dark matter, neutrinos, etc. — not everything in the Universe is matter.
For example, we also have radiation: quantized into individual entities, like matter, but massless, and with its energy defined by its wavelength. As the Universe expands, and as light travels through the expanding Universe, not only does the volume increase while the number of particles remains the same, but each quantum of radiation experiences a shift in its wavelength toward the redder end of the spectrum: longer wavelengths. This redshifting effect is how the expanding Universe was first detected, and shows up in every galaxy and quasar we measure beyond our own Local Group.
What is Dark Energy?
Meanwhile, our Universe also possesses dark energy, which is a form of energy that isn’t in the form of particles at all, but rather appears to be inherent to the fabric of space itself. While we cannot measure dark energy directly the same way we can measure the wavelength and/or energy of photons, there is a way to infer its value and properties: by looking at precisely how the redshifting light from distant objects evolves as a function of distance. Remember that there’s a relationship between the different forms of energy in the Universe and the expansion rate. When we measure the distance and redshift of various objects throughout cosmic time, they can inform us as to how much dark energy there is, as well as what its properties are. What we find is that the Universe is about ⅔ of dark energy today and that the energy density of dark energy doesn’t change: as the Universe expands, the energy density remains constant.
Mapping Cosmic History
When we put the full picture together from all the different sources of data that we have, a single, mostly consistent picture emerges. Our Universe today is expanding at somewhere around 70 km/s/Mpc, which means that for every megaparsec (about 3.26 million light-years) of distance, an object is separated from another object, the expanding Universe contributes a redshift that’s equivalent to a recessional motion of 70 km/s.
That’s what it’s doing today, mind you. But by looking to greater and greater distances and measuring the redshifts there, we can learn how the expansion rate differed in the past, and hence, what the Universe is made of: not just today, but at any point in history. Today, our Universe is made of the following forms of energy:
- about 0.008% radiation in the form of photons, or electromagnetic radiation,
- about 0.1% neutrinos, which now behave like matter but behaved like radiation early on, when their mass was very small compared to the amount of (kinetic) energy they possessed,
- about 4.9% normal matter, which includes atoms, plasmas, black holes, and everything that was once made of protons, neutrons, or electrons,
- about 27% dark matter, whose nature is still unknown but which must be massive and clumps, clusters, and gravitates like matter,
- and about 68% dark energy, which behaves as though it’s energy inherent to space itself.
Will the Universe Ever Stop Expanding?
Space, contrary to what you might think, isn’t some measurable, physical substance that you can treat the same way you’d treat particles or some other form of energy that would appear inside a detector. Instead, space is simply the backdrop — a stage, if you will — against or upon which the ‘play’ of the Universe unfolds. We can use the measurements of particles and quanta we can detect from all over the Universe to infer the properties of space, and under the rules of general relativity, if we can know what’s present within that space, we can predict how space will curve and evolve. That curvature and that evolution will then determine the future trajectory of every quantum of energy that exists, including all the quanta we’re capable of detecting.
All of the radiation within our Universe, including every gravitational wave, behaves as though space is stretching, although space itself isn’t getting any thinner. The dark energy within our Universe behaves as though new space is being created, although there’s nothing, we can measure to detect this creation. In reality, general relativity can only tell us how space behaves, evolves, and affects the energy within it; it cannot fundamentally tell us what space actually ‘is’ in a philosophical sense.
Yet, amidst this cosmic ballet, the question remains: how does the Universe expand? The answer lies not in abstract theories or mathematical conjectures but in the tangible changes we observe in the cosmos. As the Universe expands, the volume of space occupied by ‘stuff’ grows a simple yet profound testament to the dynamic nature of our cosmic home.
